The invention relates to a polymer powder which comprises at least one polymer and at least one flame retardant comprising ammonium polyphosphate, to a process for preparing this powder, and also to moldings produced by layer-by-layer application and fusion of this powder.
Very recently, a requirement has arisen for the rapid production of prototypes. The flexibility of processes which apply a pulverulent material layer-by-layer and selectively melt or bond this material makes these processes of particular interest.
Selective laser sintering is a process particularly well suited to rapid prototyping. In this process, polymer powders in a chamber are selectively irradiated briefly with a laser beam, resulting in melting of the particles of powder on which the laser beam falls. The molten particles coalesce and rapidly solidify again to give a solid mass. Three-dimensional bodies, including those of complex shape, can be produced simply and rapidly by this process, by repeatedly applying fresh layers and irradiating these.
The process of laser sintering (rapid prototyping) to realize moldings made from pulverulent polymers is described in detail in the patent specifications U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers can be employed for this application, e.g. polyacetate, polypropylene, polyethylene, ionomers, and polyamide.
Polyamide-12 powder (PA 12) has proven particularly successful in industry for laser sintering to produce moldings, in particular to produce engineering components. The parts manufactured from PA 12 powder meet the high requirements demanded with regard to mechanical loading, and therefore have properties particularly close to those of the mass-production parts subsequently produced by extrusion or injection molding.
A PA 12 powder with good suitability here has a median particle size (d50) of from 50 to 150 μm, and is obtained as in DE 197 08 946 or else DE 44 21 454, for example. It is preferable here to use a polyamide-12 powder whose melting point is from 185 to 189° C., whose enthalpy of fusion is 112±17 J/g, and whose solidification point is from 138 to 143° C., as described in EP 0 911 142.
Other processes with good suitability are the SIB process, as described in WO 01/38061 or EP 1 015 214. The two processes operate using infrared heating over an area to melt the powder, and selectivity of the melting is achieved in the first process by applying an inhibitor, and in the second process by way of a mask. Another process which has found wide acceptance in the market is 3D printing, as in EP 0 431 924; where the moldings are produced by curing of a binder applied selectively to the powder layer. Another process is described in DE 103 11 438. In this, the energy required for the fusion process is introduced by way of a microwave generator, and selectivity is achieved by applying a susceptor.
For the rapid prototyping or rapid manufacturing processes (RP or RM processes) mentioned, use may be made of pulverulent substrates, in particular polymers or copolymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, copolyester, copolyamides, terpolymers, acrylonitrile-butadiene-styrene copolymers (ABS), or a mixture of these.
Although the properties of the known polymer powders are already good, moldings produced using these powders still have some disadvantages. Particular disadvantages with the polymer powders currently used are their ready flammability and combustibility. This currently inhibits the use of the abovementioned processes in short production runs, for example in aircraft construction.